In recent years, there have been proposed and developed various types of hybrid drive systems for use of a hybrid vehicle. In any hybrid drive system, on the occasion of the mode change between the electric drive (EV) mode and the hybrid drive (HEV) mode, the mode change control has to be carried out in accordance with not only an operation condition of the vehicle such as a vehicle speed or desired driving force, but also power supply to the motor/generator or state of charge of a battery that is responsible for storing electric power generated by the motor/generator.
For example, in a case where the state of charge of the battery drops to a certain extent due to the drive of the electric drive (EV) mode, in order to prevent a situation in which restoration of the battery becomes impossible by the further drop of the battery charge state, by shifting from the electric drive (EV) mode to the hybrid drive (HEV) mode, the motor/generator is applied with an electrical generating load and the engine drives this load; then the generated electric power is stored in the battery. In a case where the battery charge state exceeds a certain degree during the drive of the hybrid drive (HEV) mode, in order to prevent deterioration of the battery by the further overcharge of the battery, the above overcharge is prevented by shifting from the hybrid drive (HEV) mode to the electric drive (EV) mode.
Accordingly, in a case where a judgment of a desired mode change from the electric drive (EV) mode to the hybrid drive (HEV) mode and a judgment of a desired mode change from the hybrid drive (HEV) mode to the electric drive (EV) mode are carried out based on the same battery charge state set value, a hunting of the control occurs when the battery charge state is around the set value. Therefore, the mode change between the electric drive (EV) mode and the hybrid drive (HEV) mode frequently occurs, and this start and stop of the engine is burdensome and/or complicated.
To solve such problems, as disclosed in Japanese Patent Provisional Publication No. 2004-023959, a relatively low first set battery charge state for changing the mode from the electric drive (EV) mode to the hybrid drive (HEV) mode and a relatively high second set battery charge state for judging the mode change from the hybrid drive (HEV) mode to the electric drive (EV) mode are individually set, and the mode change judgment is made based on these set battery charge states.
A further explanation is made with reference to FIGS. 14A-14C. These show operation time charts of a case where, during the drive of the EV mode, a battery charge state SOC becomes lower than a first set battery charge state SOC(L) at time t0. As a result the change from EV mode to the HEV mode is then carried out. After time t1, an accelerator opening APO is kept at 0 by release of an accelerator pedal with a vehicle speed VSP desired to be lowered as shown in the drawing. By the HEV mode, the battery charge state SOC rises and becomes higher than or equal to a second set battery charge state SOC(H) at time t2. As a result, the change from HEV mode to the EV mode is then carried out.
According to this mode change control, there is a hysteresis area or domain between the first set battery charge state SOC(L) and the second set battery charge state SOC(H). The EV→HEV mode change is not carried out unless the battery charge state SOC becomes lower than the first set battery charge state SOC(L). Conversely, the HEV→EV mode change is not carried out unless the battery charge state SOC becomes higher than or equal to the second set battery charge state SOC(H). Thus, the hunting of the control can be prevented, and the frequent occurrence of the mode change between the electric drive (EV) mode and the hybrid drive (HEV) mode is reduced.